The present invention relates to electrorheological fluids and devices, and a method for improving the dispersive stability of such fluids.
Electrorheological ("ER") fluids are fluids which can rapidly and reversibly vary their apparent viscosity in the presence of an applied electric field. ER fluids are generally dispersions of finely divided solids in hydrophobic, electrically non-conducting oils. They have the ability to change their flow characteristics, even to the point of becoming solid, when subjected to a sufficiently strong electrical field. When the field is removed, the fluids revert to their normal liquid state. ER fluids may be used in applications in which it is desired to control the transmission of forces by low electric power levels, for example, in clutches, hydraulic valves, shock absorbers, vibrators, or systems used for positioning and holding work pieces in position.
ER fluids have been known since 1947, when U.S. Pat. No. 2,417,508 was issued to Winslow, disclosing that certain dispersions of finely divided solids such as starch, carbon, limestone, gypsum, flour, etc., dispersed in a non-conducting liquid would undergo an increase in flow resistance when an electrical potential difference was applied. In the extensive work which has followed this discovery, many variations of ER fluids have been discovered, in which the solid phase, the liquid phase, or other components have been varied. One feature of many ER fluids is that a dispersant (also referred to as a surfactant) is required in order to maintain the finely divided solids dispersed through the liquid medium. The use of a dispersant, however, has been reported to lead to diminished electro-rheological activity in some systems.
Among the various attempts to provide an improved ER fluid are the following:
Japanese application 03/170600 (Tonen Corp.), Jul. 24, 1991, discloses an electro-viscous fluid comprising an electric insulating fluid, porous solid particles, a dispersant, and a polyhydric alcohol. The dispersants can include sulfonates, phenates, phosphonates, succinimides, amine, and nonionic dispersants including e.g. sorbitan monooleate.
Japanese application 04/120194 (Tonen Corp.), Apr. 21, 1992 (available as Derwent Abstract 92-180972/22), discloses electroviscous fluid containing at least one of partially etherified and esterified products of polyhydric alcohols in a base electroviscous fluid consisting of an electrically insulating fluid, porous solid particles, and dispersant. Dispersants include sulfonates, phenates, phosphonates, succinic imides, amines, and non-ionic dispersants.
European publication 395 359 (Tonen Corp.), Oct. 31, 1990, discloses an electrically insulating medium containing dispersed solid particles, an acid, base, or salt, a polyhydric alcohol, an antioxidant, and optionally an agent to assist dispersing of the solid particles (e.g. a sulfonate, phenate, phosphonate, succinic acid imide, amine or non-ionic dispersing agents).
European Application 342,041 (Toa Nenryo), Nov. 15, 1989, discloses an electrically insulating liquid, a porous solid particulate matter, water, and acid, base, or salt. A dispersant can also be used, for example, non-ionic dispersants such as sulfonates, phenates, phosphonates, succinic acid imides, and amines.
U.S. Pat. No. 2,970,573, Westhaver, Jul. 20, 1976, discloses electroviscous fluids comprising particles of modified starch dispersed in high concentration in a dielectric oil, the particles containing an electrolyte. Dispersants are also disclosed, usually of the water-in-oil type.
U.S. Pat. No. 3,367,872, Martinek et al., Feb. 6, 1968, discloses an electroviscous fluid comprising a non-polar oleaginous vehicle, such as a mineral oil, a particulate solid, and optionally other ingredients such as a surface active agent. Nonionic agents include ethers and esters formed by reaction of ethylene oxide with a variety of compounds such as fatty alcohols, alkyl phenols, glycol ethers, fatty acids, [etc.].
It has now been found that a certain class of dispersant imparts good dispersive stability to ER active particles in carbon-based fluids, while providing a fluid which maintains good ER activity.